136 ANNUAL REPORT SMITHSONIAN INSTITUTION, 19 34 



The catalysts of a living cell are the enzymic structures which 

 display their influences at the surface of colloidal particles or at 

 other surfaces within the cell. Current research continues to add to 

 the great number of these enzymes which can be separated from, or 

 recognized in, living cells and tissues, and to increase our knowledge 

 of their individual functions. 



A molecule within the system of the cell may remain in an inactive 

 state and enter into no reactions until at one such surface it comes 

 in contact with an enzymic structure which displays certain adjust- 

 ments to its own structure. While in such association the inactive 

 molecule becomes (to use a current term) " activated," and then 

 enters on some definite path of change. The one aspect of enzymic 

 catalysis which for the sake of my theme I wish to emphasize is its 

 high specificity. An enzyme is in general adjusted to come into 

 effective relations with one kind of molecule only, or at most with 

 molecules closely related in their structure. Evidence based on 

 kinetics justifies the belief that some sort of chemical combination 

 between enzyme and related molecule precedes the activation of the 

 latter, and for such combinations there must be close correlation in 

 structure. Many will remember that long ago Emil Fischer recog- 

 nized that enzymic action distinguishes even between two optical 

 isomers and spoke of the necessary relation being as close as that of 

 key and lock. 



There is an important consequence of this high specificity in bio- 

 logical catalysis to which I will direct your special attention. A liv- 

 ing cell is the seat of a multitude of reactions, and in order that it 

 should retain in a given environment its individual identity as an 

 organism, these reactions must be highly organized. They must be 

 of determined nature and proceed mutually adjusted with respect to 

 velocity, sequence, and in all other relations. They must be in 

 dynamic equilibrium as a whole and must return to it after disturb- 

 ance. Now if of any group of catalysts, such as are found in the 

 equipment of a cell, each one exerts limited and highly specific influ- 

 ence, this very specificity must be a potent factor in making for 

 organization. 



Consider the case of any individual cell in due relations with 

 its environment, whether an internal environment as in the case of 

 the tissue cells of higher animals, or an external environment as 

 in the case of unicellular organisms. Materials for maintenance of 

 the cell enter it from the environment. Discrimination among 

 such materials is primarily determined by permeability relations, 

 but of deeper significance in that selection is the specificity of the 

 cell catalysts. It has often been said that the living cell differs 

 from all nonliving systems in its power of selecting from a hetero- 



